For example, neurons in the medial superior olivary nucleus have medial and lateral dendrites. The synapses on the medial dendrites are largely excitatory, and they originate from the contralateral ventral cochlear nucleus. Those on the lateral dendrites are mostly inhibitory and come from the ipsilateral ventral cochlear nucleus. Differences in the phase of the sound reaching the two ears affect the strength and timing of the excitation and inhibition reaching a particular medial superior olivary neuron. The lateral superior olivary nucleus processes differences in the sound intensity that reaches the two ears to provide information about the source of the sound. The activity of superior olivary neurons can provide information about sound localization.
Several features of the primary auditory cortex resemble those of other primary sensory areas. Not only are sensory maps—in this case, tonotopic maps—present in the auditory cortex but also this cortical region performs feature extraction. Neurons in the primary auditory cortex form isofrequency columns (in which the neurons in the column have the same characteristic frequency), and they also form alternating columns, known as summation and suppression columns. Neurons in summation columns are more responsive to binaural than to monaural input. Neurons in suppression columns are less responsive to binaural than to monaural stimulation, and, accordingly, the response to one ear is dominant. Some neurons are selective for the direction of frequency change.
Bilateral lesions of the auditory cortex have some effect on the ability to distinguish the frequency or intensity of different sounds, and they reduce the abilities to localize sound and to understand speech. Unilateral lesions, however, have little effect, especially if the nondominant (for language) hemisphere is involved (see ). Evidently, frequency discrimination depends on activity at lower levels of the auditory pathway, possibly the inferior colliculus.
Twosimpletestsareoftenusedclinicallytodistinguishthemostimportanttypesofdeafness:conduction loss andsensorineural loss. Conductionhearinglossoccursindisordersoftheexternalear(e.g.,earcanalblockedbycerumen)ormiddleear(e.g.,ruptureoftheeardrum).Sensorineuralhearinglossreflectsdisordersoftheinnerear,thecochlearnerve,orcentralconnections.
TheWeber test isusedtoevaluatethemagnitudeofconductionhearingloss.Inthistest,thebaseofavibratingtuningforkisplacedagainstthemiddleoftheperson’sforehead,andthepersonisaskedtolocalizethesound.Normally,thesoundisnotlocalizedtoaparticularear.However,ifthepersonhasconductivehearingloss(e.g.,duetoapuncturedtympanicmembrane,fluidinthemiddleear,otosclerosis,orlossofcontinuityoftheossicularchain),thesoundislocalizedtothedeafearbecauseitisconductedtothecochleathroughbone.Thesoundisalsoconductedtothecochleaoftheundamagedear,butbone-conductedsounddoesnotactivatetheorganofCortiaswellasdoessoundconductednormallythroughthetympanicmembraneandossiclechain.OnereasonwhythesoundintheWebertestisnotlocalizedtothenormalearmaybethathearinginthenormalearisinhibitedbytheambientsoundlevel(auditory masking). Conversely,inpeoplewithsensorineuralhearingloss(e.g.,duetodamagetotheorganofCorti,thecochlearnerve,orthecochlearnuclei),thesoundislocalizedtothenormalside.